Aqueous dispersions containing copolymers of vinyl esters and ethylene are well known for the production of adhesives for use in the wood, paper and packaging industries. Emulsion copolymers of vinyl acetate and ethylene, i.e., VAE copolymer emulsions, are especially useful as the binder component in such adhesive products.
One application where VAE-based emulsion copolymers find utility is in the area of adhesives for tobacco products and tobacco product packaging. Such adhesives, for example, are frequently used as a sideseam adhesive to secure cigarette paper around the tobacco rod in cigarettes. These adhesives can also be used in “tipping” applications wherein the adhesive secures a filter, holder or other type of tip to one end of the cigarette. VAE-based adhesives are desirable for such applications because of their good adhesion, wet tack and shear resistance properties. VAE emulsions used in adhesive formulations also have very low odor characteristics such that their use in cigarettes and cigarette packaging does not interfere with the cigarette smell or taste.
Industry standards and in some instances government regulations require that the emulsion copolymers in tobacco product or food product adhesives be very low in residual monomer content and content of volatile organic compounds (VOCs). One adhesive component which is frequently singled out for specific control and minimization is formaldehyde. This is because formaldehyde can be produced by decomposition or disassociation of some components which are conventionally used in emulsion polymerization procedures.
Formaldehyde-generating components can include, for example, commonly employed self cross-linking co-monomers such as N-methylolacrylamide used in the copolymers which are prepared. Free formaldehyde can also be released by certain reducing agents such as sodium formaldehyde sulfoxylate used in the polymerization initiator systems employed. Certain emulsion stabilizers used, including, for instance, some types of ethylene oxide-containing surfactants can also form formaldehyde upon oxidation. Formaldehyde formation from polyoxyethylene nonionic surfactants is described, for example, in Bergh et al; Contact Dermatitis, Vol. 39, pp. 14-20 (1998).
One method for reducing the free formaldehyde content of copolymer emulsions having formaldehyde produced from materials used in the polymerization process is to introduce a formaldehyde binding or scavenging agent. U.S. Pat. Nos. 4,472,165; 4,525,492 and 5,143,954 and European Patent No. 647658B disclose the use of urea, amines and numerous derivatives of such materials as formaldehyde binders or scavengers in various resin products. Commercially available products, such as Celvolit® 149 LV marketed by Celanese, use urea as a formaldehyde scavenger.
Another approach to overcoming or ameliorating the effects of free formaldehyde released by N-methylol-based self-crosslinking co-monomers in emulsion copolymer dispersions focuses on the redox initiator systems used to prepare such dispersions. In particular, formaldehyde-generating reducing agents in such redox systems can be replaced with other types of reducing agents which do not generate formaldehyde. U.S. Pat. No. 5,540,987, for example, discloses a redox initiator system for vinyl acetate/N-methylol crosslinking co-monomer emulsion binders, which initiator system reduces free formaldehyde content of the resulting copolymer emulsion. The redox system of the '987 patent comprises a hydrophobic hydroperoxide oxidizing agent in combination with an ascorbic acid or derivative thereof as a reducing agent.
Similar technology is also disclosed in U.S. Pat. No. 6,787,594. The '594 patent describes reduced formaldehyde nonwoven binders based on emulsifier-stabilized vinyl acetate/ethylene emulsion copolymers. Such copolymers also contain an N-methylol-based crosslinking co-monomer. Formaldehyde reduction in the binders of the '594 patent is provided by using a selected type of initiator system during polymerization of the emulsion copolymer. In particular, the selected type of initiator system disclosed in the '594 patent is a redox system comprising an oxidizing agent, such as a hydrophobic peroxide, and a reducing agent which is a glycol adduct of sodium sulfite. Particularly preferred reducing agents are said to be sulfinic acid compounds such as 2-hydroxy-2-sulfinato-acetic acid-disodium salt. Reducing agents of this preferred type are sold, for example, under the tradename Bruggolite® FF-6.
Another reference which discloses preparation of emulsion polymers using a similar initiator system, and generally also using emulsifiers as stabilizing agents, is U.S. Pat. No. 6,696,519. The '519 patent utilizes a redox initiator system comprising a water-soluble oxidizing agent, a water-insoluble oxidizing agent and a reducing agent derived from sulfinic acid. The '519 patent is concerned with reducing residual monomer levels in the resulting emulsion and does not mention preparation of low formaldehyde emulsion polymers.
Notwithstanding the foregoing prior art dealing with emulsion polymers and copolymers of low formaldehyde content and emulsion polymers prepared using selected types of oxidizing and reducing agents in a redox initiator system, it would be advantageous to identify still other emulsion copolymer dispersions (and preparation processes for such dispersions) which can be used in adhesives requiring very low formaldehyde levels.